奈米結構對氮化鎵奈米柱發光二極體內部量子效率之探討

隨著化合物半導體的快速發展，氮化鎵由於其良好的材料特性，在固態照明相關領域受到了極大的關注。身為其應用之一的氮化鎵奈米柱發光二極體，更被認為具有近一步提升發光特性的潛力。然而由於諸多條件的限制，造成了氮化鎵奈米柱發光二極體元件特性不佳及發展上的困難，例如：逆偏壓下漏電流過大；發光效率低落；以及缺乏有效率的奈米結構製程技術等等，使其難以產業化。在本篇論文中，我們發展一種嶄新且具實用價值之奈米小球微影術用以製作具有p-i-n結構之奈米柱陣列，並以電漿輔助化學氣相沉積法成長二氧化矽做為奈米柱側壁保護層，接著利用化學機械研磨方式去除包覆於奈米柱頂端之二氧化矽以利後續金屬接觸層之製作。藉由本文提出之方法，可有效解決以往奈米柱發光二極體漏電流過大及製程成本高之問題。在電性上，我們的奈米柱元件在-5V偏壓下僅有4.77nA之漏電流，其理想因子(ideality factor)約為7.35；於注入電流密度約32A/cm2時，有高達6807mW/cm2之發光強度。此結果顯示我們可利用奈米小球微影術及化學機械研磨法兩種低成本且簡易之方式製作出高效能之氮化鎵奈米柱發光二極體陣列。接著我們針對藍光奈米柱發光二極體在不同溫度及電流下之電激發光頻譜特性做分析，並與平面傳統結構之元件做比較。我們發現於室溫下隨著電流增加，奈米柱元件之光子能量近乎維持常數，而平面結構元件卻有藍移之趨勢，顯示出此奈米柱結構能藉由應力釋放，有效抑制應力所造成之量子侷限史塔克效應(quantum confined Stark effect)，同時提升發光二極體之內部量子效率。另一方面，從低溫量測之結果可觀察到奈米柱結構中應力釋放效應以及側壁蝕刻生成缺陷(etching-induced defect)兩者之共存，因此我們設計兩種不同深度之奈米柱元件並進一步探討這兩效應間之相互關係。由於結合機制會隨著不同電流大小切換，兩元件總外部量子效率之變化得以清楚區分：低電流下由缺陷中之非輻射載子結合主導；高電流時則受到量子侷限史塔克效應之影響較深。對於較長之奈米柱而言，其應力釋放之效應較強但同時也具有較多之缺陷分佈，造成了總效率之緩慢增加以及相對輕微之效率下降效應；而較短的奈米柱則呈現了快速增加的總效率以及較為劇烈的下降效應。另外我們也藉由低溫環境來排除部分缺陷之非輻射結合因素。在低溫下，其量子效率之變化更可進一步說明兩效應之相互影響。由實驗結果可知，即使缺陷的多寡仍然為影響整體發光效率的關鍵因素，但無可置疑的，較長的奈米柱可確實地藉由應力釋放達到較高的內部量子效率，此意味著若能改善奈米柱蝕刻機制以減少缺陷分佈，即可實現高效能奈米柱發光二極體。

關鍵字

氮化鎵；奈米柱；發光二極體；內部量子效率；應力釋放；量子侷限史塔克效應；化學機械研磨法

並列摘要

With the tremendous growth of compound semiconductors, gallium nitride (GaN) has attracted considerable attentions in the field of solid state lighting due to the superior optical and electrical characteristics. As one of its applications, GaN-based nanorod light emitting diodes (LEDs) have been regarded to have the potential for improving optical properties. Nevertheless, there are some issues, such as the lack of efficient nano-fabrication, large leakage current under reversed bias and low optical output efficiency, that limit the performance and simplicity of manufacturing GaN nanorod LEDs. In this thesis, we demonstrated a novel and practical approach to fabricate InGaN/GaN nanorod LED arrays with p-i-n structure using nanosphere lithography for nanorod formation, PECVD (plasma-enhanced chemical vapor deposition) grown SiO2 layer for sidewall passivation, and chemical mechanical polishing (CMP) process for parallel metal contact. With such a nano-device, we achieve a reverse leakage current of 4.77nA at -5V, an ideality factor of 7.35, and an optical output intensity 6807mW/cm2 at the injection current density of 32A/cm2. Based on the high performance, the temperature and current dependent electroluminesence (EL) of blue planar and nanorod LEDs was compared over a wide temperature range. With the nearly constant photon energy at room temperature, it reveals that the nanorod structures can effectively mitigate the strain induced quantum confined Stark effect (QCSE) and improve the internal quantum efficiency (IQE) of GaN LEDs. From the result of low temperature EL, the coexistence of strain relaxation and etching-induced defect states was observed in our nano-structured devices. Thus we further explore the correlation between these effects by two samples with different length of nanorods. Since the dominant recombination mechanism is dependent on the injection current, the variation of external quantum efficiencies (EQEs) between two devices can be clarified and explained by the defect-state-induced nonradiative recombination and the mitigation of strain-induced QCSE. Longer nanorods may cause a stronger strain relaxation but more defect state distribution, resulting in a mildly increasing EQE with less droop. As we excluded the effect of defect states at low temperature, the IQE characteristics were further verified. While the influence of defect-state-induced nonradiative recombination still dominates the overall performance, it is fact that the longer nanorod will contribute to the increase of IQE. With an optimized nanorod etching mechanism, high performance LEDs with long nanorods can thus be realized soon.

並列關鍵字

GaN ； nanorod ； LED ； internal quantum efficiency ； strain relaxation ； quantum confined Stark effect ； chemical mechanical polishing

參考文獻

[1] X. Cao, S. LeBoeuf, L. Rowland, C. Yan and H. Liu 2003 Temperature-dependent emission intensity and energy shift in InGaN/GaN multiple-quantum-well light-emitting diodes Applied Physics Letters 82 3614

[2] H. Chang, Y. Hsieh, T. Chen, Y. Chen, C. Liang, T. Lin, S. Tseng and L. Chen 2003 Strong luminescence from strain relaxed InGaN/GaN nanotips for highly efficient light emitters Appl. Phys. Lett 82 880-2

[3] H. J. Chang, Y. P. Hsieh, T. T. Chen, Y. F. Chen, C. T. Liang, T. Y. Lin, S. C. Tseng and L. C. Chen 2007 Strong luminescence from strain relaxed InGaN/GaN nanotips for highly efficient light emitters Opt. Express 15 9357-65

[4] C. H. Chen, W. H. Chen, Y. F. Chen and T. Y. Lin 2003 Piezoelectric, electro-optical, and photoelastic effects in In[sub x]Ga[sub 1 - x]N/GaN multiple quantum wells Applied Physics Letters 83 1770-2

[5] H. Chen, D. Yeh, Y. Lu, C. Chen, C. Huang, T. Tang, C. Yang and C. Wu 2006 Strain relaxation and quantum confinement in InGaN/GaN nanoposts Nanotechnology 17 1454

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奈米結構對氮化鎵奈米柱發光二極體內部量子效率之探討

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